FIG. 2 is a sectional view showing a general configuration example of a microphone unit that a conventional condenser microphone has. This microphone unit 10B is unidirectional, and in this example, there is shown a microphone unit that is attachable to and detachable from a microphone body, not shown, (exchangeable), which microphone unit is applied to tie clip microphones, gooseneck microphones, and the like.
The microphone unit 10B includes a cylindrical unit case 20 formed of, for example, a brass alloy. In the unit case 20, a diaphragm and a backplate are arranged opposedly via a spacer ring (all of the three elements are not shown) as well known, and an electrostatic acoustoelectric converter 30 for converting coming sound waves into electrical signals is housed.
Since the microphone unit 10B is unidirectional, the unit case 20 is provided with a rear audio terminal (rear sound wave introduction port) 22, which takes in velocity components, on the side surface side thereof in addition to a front audio terminal (front sound wave introduction port directed to a sound source) 21 provided on the front surface thereof.
Usually, in the unit case 20, a metal mesh 40 for covering the rear audio terminal 22 from the inside is provided. This metal mesh 40 is provided to inhibit foreign matters from intruding into the unit case 20 from the rear audio terminal 22. The metal mesh 40 is brought into contact with the inner wall surface of the unit case 20 by the spring property of the metal mesh 40 itself, and is fixed by using an adhesive to prevent the metal mesh 40 from coming off due to vibrations etc. For example, Patent Document 1 (Japanese Patent Application Publication No. S55-105492) or Patent Document 2 (Japanese Patent Application Publication No. S56-43985) should be referred to.
On the inner surface side of the front audio terminal 21 as well, a metal mesh for inhibiting the intrusion of foreign matters is provided in the same way, but the illustration of this metal mesh is omitted.
From the acoustoelectric converter 30, a signal draw-out electrode 31 connected to the backplate is drawn out. Along with the connection of the microphone unit 10B to the microphone body, the signal draw-out electrode 31 is connected to a sound output circuit, not shown, in the microphone body. Since the acoustoelectric converter 30 has a very high impedance, an impedance converter is provided on the input side of the sound output circuit.
A vacuum tube is used as the impedance converter on rare occasions. In most cases, however, a field effect transistor (FET) is used as the impedance converter. In this case, the signal draw-out electrode 31 is connected to the gate electrode of the FET, and the sound output circuit is connected to between the source and the drain of the FET.
Since the impedance converter of this type also acts as a wave detecting device, if a high-frequency current caused, for example, by electromagnetic waves is applied to the microphone unit 10B as disturbance, the current is detected by the impedance converter, and thereby noise of audio frequency is generated. The noise of this kind is scarcely generated in the case where the electromagnetic shield of the microphone unit 10B is ensured.
At the front audio terminal 21, the diaphragm is arranged so as to be opposed to the front audio terminal 21, and a metallic layer formed in the diaphragm is connected to the unit case 20, which provides grounding, via a metallic support ring (diaphragm ring), so that the electromagnetic waves intruding from the front audio terminal 21 rarely pose a problem.
However, regarding to the rear audio terminal 22, the contact between the metal mesh 40 covering the rear audio terminal 22 from the inside and the inner wall surface of the unit case 20 depends on only the spring property of the metal mesh 40 itself as described above, so that the contact point area is small, and therefore the shield property is not necessarily sufficient.
In recent years, cellular phones have come into wide use. The cellular phone emits considerably strong electromagnetic waves (for example, within the range of about several centimeters to several tens centimeters, a field intensity reaching tens of thousands times of field intensity produced in the city by commercial electric waves).
Therefore, if a cellular phone is used near the microphone, since the contact between the metal mesh 40 and the unit case 20 is insufficient, the contact portion has a high impedance in terms of high frequency, so that a high-frequency current caused by the high impedance intrudes into the microphone body, which may generate loud noise.
Also, since the contact state differs from microphone unit to microphone unit, the degree of generation of noise caused by high-frequency current varies. Also, if the opening of the rear audio terminal 22 is made large to improve the acoustic characteristics, the high-frequency current intrudes more easily.
Accordingly, an object of the present invention is to ensure the electromagnetic shield at a rear audio terminal covered by a metal mesh in a unidirectional microphone unit.